[DevTools] Rework DevToolsSession interaction with domain handlers.

content/browser/devtools/protocol/target_handler.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/devtools/protocol/target_handler.h"
 
 #include "content/browser/devtools/devtools_manager.h"
 #include "content/browser/devtools/service_worker_devtools_agent_host.h"
 #include "content/browser/frame_host/frame_tree.h"
 #include "content/browser/frame_host/frame_tree_node.h"
@@ skipping 75 matching lines @@
       .SetTargetId(host->GetId())
       .SetTitle(host->GetTitle())
       .SetUrl(host->GetURL().spec())
       .SetType(host->GetType())
       .Build();
 }
 
 }  // namespace
 
 TargetHandler::TargetHandler()
-    : discover_(false),
+    : DevToolsDomainHandler(Target::Metainfo::domainName),
+      discover_(false),
       auto_attach_(false),
       wait_for_debugger_on_start_(false),
       attach_to_frames_(false),
       render_frame_host_(nullptr) {
 }
 
 TargetHandler::~TargetHandler() {
 }
 
 void TargetHandler::Wire(UberDispatcher* dispatcher) {
@@ skipping 341 matching lines @@
   UpdateServiceWorkers();
 }
 
 void TargetHandler::WorkerDestroyed(
     ServiceWorkerDevToolsAgentHost* host) {
   UpdateServiceWorkers();
 }
 
 }  // namespace protocol
 }  // namespace content